S.
HI K.
This is a VERY common problem with kids.After being a nanny i reccommend using a star chart.Give him a star for each poo he does on the toilet and as he does so many reward with a special treat(trip out somewhere)
also take him with you as you go tothe toilet and talk him through what you are doing.Show him you are ok.
Also give foods/drinks to help produce poos so he cant hold them in a easily.
you must stop using the diaper as you are just creating more of a problem for hm.
loads of praise/excitement with the chart should do it
good luck!!